img
.
Thus
The interval includes zero. We are less than 95% certain that the two sets of measurements are
different! And indeed, this is taken from a set of runs that were done incorrectly. After modifying
the program, it was recompiled to a.out by mistake. The two sets of measurements actually
come from exactly the same binary!
If you run only four measurements, the difference between the measured means must be greater
than (1.96 x std. error), or roughly twice the measured standard deviation. By running it ten times
(see Table 15-2) the 95% confidence level is obtained when the difference is greater than the
standard error. When the numbers are reasonably close together, you can eyeball the mean and
standard error fairly easily.
Table 15-2. Runtimes for Ten Trials
Run 1
Run 2
N_PROD = 1 N_CONS = 4
N_PROD = 1 N_CONS = 5
rate: 85.965975/s
rate: 89.984372/s
rate: 86.802915/s
rate: 91.710778/s
rate: 88.528658/s
rate: 91.075302/s
rate: 85.411582/s
rate: 91.741185/s
rate: 85.957945/s
rate: 87.995095/s
rate: 84.514983/s
rate: 93.661803/s
rate: 86.732842/s
rate: 89.505427/s
rate: 84.284994/s
rate: 89.262953/s
rate: 85.024726/s
rate: 89.611914/s
rate: 85.602694/s
rate: 91.972079/s
Mean rate: 85.88/s
Mean rate: 90.65/s
Standard deviation: 1.25
Standard deviation: 1.67
The difference between the means is about 5 and the standard deviation is about 1.5. The
difference, 5 1.5, is much greater than zero, so we can conclude with confidence that run 2 is
indeed superior to run 1. Doing this stuff well is not at all obvious, and doing it wrong is all too
common. We're not expecting you to do this carefully on every test, but you do have to be aware
of it.
General Performance Optimizations
By far the most important optimizations will not be specific to threaded programs, but rather, the
general optimizations you do for nonthread programs. We'll mention these optimizations but leave
the specifics to you. First, you choose the best algorithm. Second, you select the correct compiler
optimization. Third, you buy enough RAM to avoid excessive paging. Fourth, you minimize I/O.
Fifth, you minimize cache misses. Sixth, you do any other loop optimizations that the compiler
was unable to do. Finally, you can do the thread specific optimizations.
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